1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to an IPS (in-plane switching mode) liquid crystal display device and fabricating method thereof.
2. Discussion of the Related Art
Generally, cathode ray tubes (hereinafter abbreviated CRTs) have been the most popular of the various display devices that display image information on a screen. Yet, CRTs have a considerable amount of volume and weight per unit display area, thereby resulting in CRTs being inconvenient for use.
Display devices, which have been used in a limited capacity for TV and the like, are now being employed in personal computers, notebook computers, vehicle instrument boards, and electronic advertisement boards and the like. As massive volumes of data containing image information is enabled to be transferred due to the development of information communication technology, a next generation display device enabling implementation of processing the image information becomes more important.
Such a next generation display device needs to have lightweight, slimness, shortness, smallness, high luminance, wide screen, low power consumption and low price. Hence, a liquid crystal display device is an example of one of the next generation display devices.
Display resolution of the liquid crystal display (LCD) is better than that of any other flat panel display devices. In implementing moving pictures, the LCD has a response speed characteristic as fast as that of CRT.
As one of the popular LCDs, there is a TN (twisted nematic) mode LCD. In the TN mode LCD, electrodes are provided to a pair of substrates, respectively, a liquid crystal director is aligned to be twisted by 90°, and a voltage is applied to the electrodes to drive the liquid crystal director.
Yet, it is disadvantageous that the TN mode LCD has a narrow viewing angle.
To solve the problem of the narrow viewing angle, many efforts have been made to research and develop LCD devices employing various new modes. For example, there are IPS (in-plane switching mode), OCB (optically compensated birefringence mode) and the like.
In the IPS LCD, in order to drive liquid crystal molecules maintaining a horizontal state to a substrate, a voltage is applied between a pair of electrodes formed on the same substrate to generate an electric field in a direction horizontal to the substrate. Namely, a long axis of the liquid crystal molecule is not erected against the substrate.
Hence, the variation of the birefringence of liquid crystals for a viewing direction is small to have a viewing angle characteristic better than that of the related art TN LCD.
An IPS LCD device according to a related art is explained in detail with reference to the attached drawings as follows.
FIG. 1A and FIG. 1B are cross-sectional diagrams of an IPS LCD device according to a related art, in which operations in off/on states are shown, respectively.
In FIG. 1A, since a horizontal electric field is not applied in the off-state, there is no motion of a liquid crystal layer 211.
In FIG. 1B, alignment of liquid crystals in an on-state with voltage impression is shown. There is no phase change of a liquid crystal 211a at a position confronting a common electrode 217 or a pixel electrode 230. Yet, another liquid crystal 211b situated between the common electrode 217 and the pixel electrode 230 is aligned by a horizontal electric field K generated from voltage impression between the common and pixel electrodes 217 and 230 in the same direction of the horizontal electric field K.
Namely, in the IPS LCD, a viewing angle is widened due to the liquid crystals moved by the horizontal electric field.
FIG. 2 is a flowchart of a method of fabricating an IPS LCD device according to a related art.
In FIG. 2, upper and lower substrates of an IPS LCD device are prepared (S100).
Subsequently, a cleaning process S110 is carried out to remove particles on the substrates having various patterns formed thereon. An alignment layer printing process S120 for printing polyimide (PI), i.e., an alignment layer raw material liquid on an upper surface of the substrate is then carried out using an alignment layer printing instrument.
Next, an alignment layer plasticizing process S130 for hardening the alignment layer raw material liquid in a manner of heating to dry solvent of the alignment layer raw material liquid is carried out.
Subsequently, an alignment layer rubbing process S140 is carried out using a rubbing machine to form recesses by rubbing a surface of the plasticized alignment layer surface in a uniform direction.
After completion of the alignment layer forming process, a seal pattern is formed on an edge of the upper substrate except a liquid crystal inlet. And, spacers are scattered on the lower substrate (S150).
Subsequently, the upper and lower substrates are bonded to each other (S160). In doing so, since light leakage occurs in case of failing in meeting a specific margin, precision of several micrometers (μm) is needed.
A cell cutting process is then carried out to cut the bonded substrates into unit cells (S170). The cell cutting process, which is to cut the completely bonded substrates into the unit cells having a specific size each, consists of a scribing process for forming lines on surfaces of the upper and lower substrates and a breaking process for cutting the substrates by impacting the scribed lines.
Finally, liquid crystals are injected in a gap between the substrates cut into each of the unit cells and the liquid crystal inlet is then sealed to prevent the liquid crystals from leaking out (S180). Thus, the LCD device is completed.
In this case, physical properties of liquid crystals are changed according to molecule alignment state, whereby a difference in a response to an external force such as an electric field and the like takes place.
Because of the above-explained properties of liquid crystal molecules, alignment control of liquid crystal molecules is essential to a configuration of the LCD device as well as study of liquid crystal physical property.
Specifically, the rubbing process for aligning liquid crystal molecules uniformly in a uniform direction plays an important role in driving the LCD normally and deciding uniform display characteristics of a screen. Hence, many efforts have been made to research and study the rubbing process.
A process of forming an alignment layer to decide an initial alignment direction of liquid crystal molecules according to a related art is explained in detail as follows.
First of all, formation of an alignment layer is carried out in a manner of coating a polymer film and aligning an alignment layer in a uniform direction.
A polyimide-based organic substance is mainly used as the alignment layer, and the alignment layer is aligned by a rubbing method.
The rubbing method is carried out in a manner of coating a polyimide-based organic substance on a substrate, removing a solvent at 60˜80° C., hardening the organic substance at 80˜200° C. to form a polyimide alignment layer, and then rubbing the alignment layer in a uniform direction using a rubbing cloth of velvet and the like.
Such a rubbing method facilitates alignment treatment to be suitable for mass production and to guarantee stable alignment.
Yet, such a rubbing method may result in a failure of rubbing in case of performing rubbing using a roller having a defective rubbing cloth thereto.
Namely, since a rubbing method using a rubbing cloth is carried out through a direct contact between an alignment layer and the rubbing cloth, various problems such as contamination of liquid crystal cells due to particles, breakage of TFT devices previously formed on a substrate due to static electricity, necessity of additional cleaning after rubbing, non-uniformity of alignment in wide area application and the like take place to lower yield of LCD fabrication.
Moreover, unlike the TN LCD, the IPS LCD needs a small pre-tilt angle. The reason is explained as follows.
Since the IPS LCD has a smaller retardation variation according to a gray level in all viewing angles rather than the TN LCD, a viewing angle of the IPS LCD is enhanced. Yet, in case of a small pre-tilt angle, asymmetry of the viewing angle occurs in the IPS LCD.
However, if the alignment layer is formed by performing the rubbing treatment on the generally used alignment layer of polyimide and the like, the pre-tilt angle becomes high. And, it is unable to secure alignment stability of liquid crystals. Moreover, asymmetry of the viewing angle occurs.